1. Field of the Invention
This invention relates to a Digital Audio Broadcasting (DAB) system for the wide-area distribution of multiple channels of audio programming, and in particular to a DAB system and method having improved receiver performance through coding gain from code diversity and packet combining, and distributed gap filler network.
2. Description of the Prior Art
Several DAB methods and systems have been proposed for Direct Broadcast Satellite Radio that essentially broadcasts digital audio signals for reception by fixed and mobile receivers. Such DAB systems and methods heretofore have yet to overcome problems that affect the overall system performance so as to make it commercially feasible. The most significant problems in DAB systems are signal shadowing, fading, and temporal blockage. Shadowing problems are dominated by factors such as the intermittent blockage of the line of sight to the receiver from the satellite by natural or man-made objects. For example, in suburban environments tree shadowing is the prominent signal impairment, while in urban environments, buildings cause the major shadowing effect. Fading problems are mainly caused by multipath signals to the mobile receiver in suburban and urban environments. Signal fading can be mitigated by adapting temporal diversity techniques such as time diversity and spatial diversity. Conventional time diversity schemes operate on the same signal so as to randomize the outage patterns through interleaving, data repetition, and/or some form of channel coding. Conventional spatial diversity techniques operate on the same signal using a dual satellite scheme and/or an antenna diversity technique for remedying shadowing and temporal blockages.
Conventional DAB systems and methods have sought to mitigate the problems of multipath fading and foliage attenuation by employing two geosynchronous satellites. U.S. Pat. Nos. 5,319,673 and 5,278,863 to Briskman disclose a polarization diversity technique in a spread spectrum system (either direct sequence or frequency hopping schemes) that employs frequency diversity to combat fading in a frequency selective channel. A conventional Code Division Multiple access (CDMA) system, using two geosynchronous satellites and a dual diversity technique is utilized, however, the Briskman system cannot provide seamless high performance service over the entire coverage area, since conventional CDMA system performance is limited by self-interference. Self-interference is induced by the cross-correlations of the Pseudo Noise (PN) sequences used for the different program channels. Furthermore, the Briskman system employs a dual polarization approach to separate signals from two satellites, the receiver then selecting the stronger of the two broadcasts of the same signal. While a dual satellite system increases the probability that a mobile or fixed receiver has line-of-sight contact with one of at least two satellites, the unselected satellite's signal must be separated and eliminated from further signal processing, otherwise, the weaker signal acts like added noise to the stronger signal. Additionally, preserving the polarization in the mobile signal path is very difficult because the reflected signals tend to invert the polarization. Therefore, conventional DAB systems are inefficient in power and bandwidth usage whereby performance is inadequate for subscription-grade quality of service.
Finally, even with two satellites, signal blockage, shadowing, and fading problems continue to occur in urban and suburban environments. Conventional DAB systems have sought to solve this problem by employing a network of gap filler transmitters to provide the signal when both satellites are blocked from view. However, such network of gap fillers transmitting the same broadcast signal increases the self-interference problem, which in turn increases the cost and complexity of the gap filler network because of the desire to use increased transmit power levels to combat self-interference. Previous attempts to solve this problem have been inadequate for subscription grade service given the intended DAB service area (continental United States) and the pervasiveness of the shadowing/fading problem; this has created new cost problems whereby gap fillers are required in virtually all urban and suburban locations. Therefore, the DAB method and system of the present invention advantageously provides a way to reduce the number and cost of such gap fillers and a critical solution to the overall network design problem. The DAB method and system of the present invention advantageously utilizes code diversity (whereby two different non-self-interfering turbo encoded signals are transmitted and substantially combined inside the receiver) to provide improved performance through higher coding gains, fewer gap fillers, and reduced transmit power level requirements from either of each of the satellite or the gap fillers.
Other DAB systems have used dual antennas and a Viterbi-algorithm method over a fading channel to reduce the affects of signal fading in the received signal. U.S. Pat. No. 5,191,598 to Backstrom et al. discloses a system for receiving radio signals on at least two mutually spaced antennas and a receiver to process samples of the signal using a Viterbi-algorithm thereby reducing signal fading. Various antenna diversity schemes also have been used and these are characterized by the orthogonal polarization of the broadcast signals. For example, in U.S. Pat. No. 5,485,485 to Briskman, a dual antenna system is disclosed that selects the stronger of two signals having substantially the same content and frequency. The dual antenna approach requires two physical antennas on the mobile receiving station which is inconvenient and expensive as multiple antennas are installed on the vehicle's roof, and their employment illustrates the severity of the reception problems of conventional DAB systems. Additionally, the performance gain due to a dual-antenna system is minimal unless the receiver uses extensive signal processing techniques to accurately compensate for the angle of arrival. The method and system of the present invention solves these problems by advantageously utilizing RAKE receivers to combine multipath signals from satellites and gap fillers at the DAB receiver.
U.S. Pat. No. 5,544,156 to Teder et al. discloses a system and method for coherently demodulating an uplink signal in a multirate, CDMA system. However, the receiver performance of the conventional CDMA and/or Viterbi based coding systems is limited by multiple access channel interference and does not provide performance and coding gains at the level provided by the present invention. As a result, the present invention uses a Turbo Code method and system along with Orthogonal Code Division Multiple Access (OCDMA) techniques to advantageously eliminate the effects of multiple access interference and provide high coding gain, which results in robust DAB reception at lower overall power levels.
Finally, an error coding system using turbo codes, ie., codes that associate with parallel concatenation of two convolutional codes separated by an interleaver, is disclosed in U.S. Pat. No. 5,446,747 to Berrou and U.S. Pat. No. 5,406,570 to Berrou et al. While such systems have acceptable coding gain they do not address the dual-signal path case or the benefits provided through the use of code combining and code diversity. These codes do not transmit interleaved data elements, but only transmit uninterleaved data elements and parity check elements from both uninterleaved and interleaved sequences. The present invention transmits uninterleaved and interleaved data elements with corresponding coded data elements to two separate signal paths, and at the turbo decoder, combines these two elements advantageously to achieve an effective overall code rate 1/4 from the simultaneous reception and combination of two code rate 1/2 signals, thereby providing significantly improved performance.
The present invention is aimed at overcoming these different drawbacks of the prior art.